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NAVTEX decoder (PC)
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Technical information

SSB modulation
NAVTEX data format

Tools & resources

NAVTEX message encoder

SSB modulation

A carrier wave 100% modulated with a tone gives the waveform and spectrum shown below:

Where y= sin(fc)*(sin(fm)+1)

The majority of the transmitter's output power goes in to the carrier (the central peak) which carries no information. This can be suppressed at the transmitter by removing the bias from the modulation signal so that:

y= sin(fc)*sin(fm)

Leaving only the two sidebands at fc+fm and fc-fm:

This has improved the efficiency of the transmitter, but there is still twice as much power as necessary being used. The upper sideband (USB) or lower sideband (LSB) can be removed while still retaining the data capacity of the channel. This will be excellently explained when I put in a diagram of an SSB modulator - remind me. In the meantime, this is what you get if you supress the USB:

The SSB signal (fssb) can be recovered by mixing it with the output of a local oscillator running at the same frequency as the transmitter's carrier fc. This results in two frequency components, one at fc+fssb and the other at fc-fssb which contains the wanted information. The higher of the two can be easily removed by filtering as it is usually very much higher than the wanted signal. The diagram below shows this. Only the wanted signal is visible on the spectrum because fc+fssb is off the right hand end of the display.

Any discrepancy between the frequency of the receiver's local oscillator and the transmitter's carrier results in the spectrum of the received signal being shifted by the amount of the error. For speech signals this can make the speaker sound like Donald Duck. Another problem with SSB is that you may not know if you're receiving the USB or LSB and so don't know which 'side' of the signal to put your local oscillator. Getting it wrong results in the spectrum of the received signal being inverted. Speech distorted in this way is generally unintelligible. Here are some examples:

Original signal
Spectrum shifted down
Spectrum inverted

The diagram below doesn't really make sense until I've put in the system diagram of an SSB modulator above, but shows the output of an SSB transmitter where incomplete suppression of the carrier has occurred. Sometimes this 'vestigial' carrier is intentionally transmitted to provide the receiver with a reference signal to lock on to.